changed my mind on this. If you put a resistor of the same rating in parallel to the lamp it just means that you'll draw twice the current to light the lamp under normal conditions. What you need is a much higher rating parallel resistor so that when lit the lamp pulls most of the current, when blown the resistor allows the exciter on the Alternator to work but still only allows minimall current to flow and protect it. Working on the assumption that the field windings in the alternator are pretty low resistance and only need minimal current to work until the alternator starts providing its own current I'm going for 500 ohm in parallel to the lamp and a 1N4001 diode as you suggested. this way there'll be about 180mA max going down the wire under normal conditions (until the lamp goes out) and more like 26mA if the bulb goes. I had a good look around the web trying to get an independant value for that resistor and 500 ohms kept popping up as the standard.
Part of the problem with this is there's 1, 2, 3 and 4 wire alternators so trying to understand how mine works was the big hurdle and now I think I understand that, I think I know what the lamp needs. Technically the alternator would still work even without the exciter feed through the lamp but you'd have to rev the engine a bit to get it going
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